1 Field of the Invention
This invention relates to a GaP light emitting element substrate and a method of manufacturing it, and more specifically to a GaP light emitting element substrate comprising a substrate with a plurality of GaP layers on it which is used when manufacturing GaP light emitting diodes which emit green light, and a method of manufacturing it.
2. Prior Art
Light emitting elements such as light emitting diodes are obtained normally by layering a plurality of semiconductor layers on a semiconductor substrate to prepare a multi-layer semiconductor substrate with a pn junction(s), and making it into elements. Of these, green light emitting diodes can be obtained by using a light emitting element substrate prepared by forming one or more layers of both n-type and p-type GaP layers, one after another, on an n-type GaP single crystal substrate.
GaP is an indirect transition type semiconductor, and therefore the brightness is very low when a pn junction is simply formed. Therefore nitrogen (N), which would be the luminescence center, is added to the n-type GaP layer in the proximity of the pn junction to increase the light emitting output.
A light emitting diode prepared from the GaP light emitting element substrate which has the nitrogen-doped n-type GaP layer, as described above, will produce a yellow-green light emission with a peak wavelength of about 567 nm.
FIG. 1 shows an example of the cross-sectional structure of a GaP light emitting element substrate which produces a green light emission. For this light emitting element substrate, an n-type GaP buffer layer 11, an n-type GaP layer 12, a nitrogen-doped n-type GaP layer 13 and a p-type GaP layer 14 are formed one after another on an n-type GaP single crystal substrate 10.
For the method of forming each GaP layer on the n-type GaP single crystal substrate 10, the liquid phase epitaxial growth method, for example, can be used. For the liquid phase epitaxial growth , two methods are normally employed.
In the first liquid phase epitaxial growth method, for example, a Ga solution prepared by dissolving GaP poly-crystals in fused Ga at 1060.degree. C. is placed on a GaP substrate, and the GaP layer is grown by gradually lowering the temperature to precipitate GaP in the Ga solution on the GaP substrate.
In the other liquid phase epitaxial growth method (hereafter referred to as melt-back liquid epitaxial growth method or melt-back method for short), fused Ga is placed on a GaP substrate, and then, for example, the temperature is raised to 1060.degree. C. so that the upper portion of the GaP substrate is dissolved into the fused Ga to prepare the Ga solution, and then the GaP layer is grown by gradually lowering the temperature to precipitate GaP in the Ga solution on the GaP substrate.
Recently, the progress of GaP light emitting diodes which emit yellow-green light has been remarkable, and each year diodes with higher light emission outputs are developed. Along with this trend toward higher light emission output, the application range of the GaP light emitting diodes has spread to a wider range. However, development of light emitting diodes with even higher light emission outputs is desirable in order to further expand the application range.
In order to obtain a light emitting element substrate for manufacturing light emitting elements with high light emission outputs, it has become common, as described, for example, in Japanese examined patent publication (Tokko) Hei-2-18319, to prepare a multi-layer GaP substrate and use it when growing the GaP layer with the melt-back method as described above. In this method, a multi-layer GaP substrate is previously prepared by forming an n-type GaP buffer layer 11 on an n-type GaP single crystal substrate 10, and, for the next step, the melt-back method is used to dissolve the upper portion of the n-type GaP buffer layer 11 on said multi-layer GaP substrate, then GaP is precipitated again to form the n-type GaP layer 12, the nitrogen-doped n-type GaP layer 13 and the p-type GaP layer 14, one after another.